1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine of an automobile or the like.
2. Description of Related Art
Generally, a spark plug for use in an internal combustion engine of an automobile or the like includes a cylindrical housing, a cylindrical insulator held inside the housing, a center electrode held inside the insulator, and a ground electrode disposed so as to form a spark discharge gap with the center electrode. Inside the insulator, a terminal metal fitting is disposed such that its proximal end portion projects from the insulator. Between the terminal metal fitting and the center electrode within the insulator, a resistor is disposed for suppressing electrical noise. A sealing member made of copper glass, for example, is filled in a space between the resistor and the terminal metal fitting and in a space between the resistor and the center electrode.
To manufacture the above spark plug, the center electrode is put into the insulator at first. Subsequently, a copper glass powder as a material of the sealing member and a powder material of the resistor are put into the insulator from the proximal end side. Further, the copper glass material is put into the insulator form the proximal end side, and then, the terminal metal fitting is inserted into the insulator from the proximal end side. After that, these components and materials are heated to a predetermined temperature, and the terminal metal fitting is pushed toward the axial distal end side. As a result, the copper glass powder and the resistor powder material are melted to flow and adhere to the respective components within the insulator, so that the center electrode, the terminal metal fitting and the resistor are fixed, and gaps are sealed by the copper glass within the insulator.
Accordingly, the terminal metal fitting inserted into the insulator is applied with a large axial force at its small-diameter portion at the time of manufacturing the spark plug.
Recently, it is required to reduce the sizes of spark plugs to enable reducing the sizes of internal combustion engines and increasing design freedom of the internal combustion engines. Therefore, the inner diameter of the insulator, and the diameter of the small-diameter portion of the terminal metal fitting disposed in the insulator are also required to be smaller. Accordingly, the small-diameter portion may be deformed when applied with a large axial force. In this case, there is a concern that the pressing force applied to the sealing member and the resistor by the terminal metal fitting may be insufficient, or varies greatly, causing the adhesion of the sealing member to the center electrode, the terminal metal fitting and the resistor to be insufficient.
Japanese Patent Application Laid-open No. 2013-41753 proposes to specify dimensions of the terminal metal fitting, for example, the outer diameter and length of the leg part of the terminal metal fitting, to overcome the above problem.
However, if the dimensions of the terminal metal fitting are specified and fixed, the design freedom of the terminal metal fitting and accordingly the design freedom of the spark plug are lowered. For example, this may cause a problem that it is difficult to design spark plugs having various shapes, for example the so-called long-reach spark plug having a large axial length.